Custom placement policies for virtual machines

ABSTRACT

A component of a computing service obtains respective indications of placement policies that contain host selection rules for application execution environments such as guest virtual machines. With respect to a request for a particular application execution environment, a group of applicable placement policies is identified. A candidate pool of hosts is selected using the group of placement policies, and members of the pool are ranked to identify a particular host on which the requested application execution environment is instantiated.

BACKGROUND

Many companies and other organizations operate computer networks thatinterconnect numerous computing systems to support their operations,such as with the computing systems being co-located (e.g., as part of alocal network) or instead located in multiple distinct geographicallocations (e.g., connected via one or more private or publicintermediate networks). For example, data centers housing significantnumbers of interconnected computing systems have become commonplace,such as private data centers that are operated by and on behalf of asingle organization, and public data centers that are operated byentities as businesses to provide computing resources to customers. Somepublic data center operators provide network access, power, and secureinstallation facilities for hardware owned by various customers, whileother public data center operators provide “full service” facilitiesthat also include hardware resources made available for use by theircustomers.

The advent of virtualization technologies for commodity hardware hasprovided benefits with respect to managing large-scale computingresources for many customers with diverse needs, allowing variouscomputing resources to be efficiently and securely shared by multiplecustomers. For example, virtualization technologies may allow a singlephysical virtualization host to be shared among multiple users byproviding each user with one or more virtual machines hosted by thesingle virtualization host. Each such virtual machine may represent asoftware simulation acting as a distinct logical computing system thatprovides users with the illusion that they are the sole operators of agiven hardware computing resource, while also providing applicationisolation and security among the various virtual machines. Instantiatingseveral different virtual machines on the same host may also helpincrease the overall hardware utilization levels at a data center,leading to higher returns on investment. At the same time, if too manyvirtual machines are set up on a given physical host to support desiredlevels of application performance, customer satisfaction levels maydrop.

Over time, as new types of hardware servers are deployed at avirtualized computing service, as existing servers or data centersbecome less suitable for various categories of virtual machines, and asapplications with a wider range of service level requirements are runusing the service, the number of factors which have to be taken intoaccount to select an appropriate virtualization host for a givenclient's virtual machine request may increase dramatically. Furthermore,the virtual machine placement preferences or requirements of one clientof the virtualized computing service may differ from those of another.In large virtualization networks with hundreds of thousands of clients,numerous types of hosts, and numerous categories of supported virtualmachines, virtual machine placement decisions may present a non-trivialchallenge.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example system environment in which customizedplacement policies for guest virtual machines may be used at a virtualcomputing service, according to at least some embodiments.

FIG. 2 illustrates example elements of a placement policy for guestvirtual machines, according to at least some embodiments.

FIG. 3 illustrates examples of location-based applicability criteria forplacement policies, according to at least some embodiments.

FIG. 4 illustrates examples of supported categories of guest virtualmachines for which respective placement policies may be generated,according to at least some embodiments.

FIG. 5 illustrates examples of placement policies applicable toclient-specific collections of guest virtual machines, according to atleast some embodiments.

FIG. 6 illustrates examples of factors that may be taken into account toselect members of candidate pools of virtualization hosts for placing aguest virtual machine, and to rank members of such candidate pools,according to at least some embodiments.

FIG. 7 illustrates an example web-based programmatic interface which maybe used to generate or edit placement policies for guest virtualmachines, according to at least some embodiments.

FIG. 8 illustrates the use of policy caches at nodes of a distributedplacement manager, according to at least some embodiments.

FIG. 9 illustrates an example policy-driven placement manager which,while implemented using resources of a provider network, may be used forplacement decisions at a client network, according to at least someembodiments.

FIG. 10 illustrates an example policy-driven placement manager which maybe executed using resources of a client network, according to at leastsome embodiments.

FIG. 11 illustrates aspects of operations that may be performed tosupport customized policies for selecting virtualization hosts forvirtual machines, according to at least some embodiments.

FIG. 12 is a block diagram illustrating an example computing device thatmay be used in at least some embodiments.

While embodiments are described herein by way of example for severalembodiments and illustrative drawings, those skilled in the art willrecognize that embodiments are not limited to the embodiments ordrawings described. It should be understood, that the drawings anddetailed description thereto are not intended to limit embodiments tothe particular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope as defined by the appended claims. The headings usedherein are for organizational purposes only and are not meant to be usedto limit the scope of the description or the claims. As used throughoutthis application, the word “may” is used in a permissive sense (i.e.,meaning having the potential to), rather than the mandatory sense (i.e.,meaning must). Similarly, the words “include,” “including,” and“includes” mean including, but not limited to. When used in the claims,the term “or” is used as an inclusive or and not as an exclusive or. Forexample, the phrase “at least one of x, y, or z” means any one of x, y,and z, as well as any combination thereof.

DETAILED DESCRIPTION

Various embodiments of methods and apparatus for generating and applyingcustomizable placement policies for virtual machines are described. Thetechniques and algorithms may be used to select target virtualizationhosts to be used for virtual machines in a variety of contexts or usagemodes. In some embodiments, the techniques may be implemented atcontrol-plane components of a virtualized computing service (VCS), andmay be used to select virtualization hosts from the VCS's fleet of hostsfor new guest virtual machines requested by clients of the VCS. In otherembodiments, the placement policies may be applied for virtualizationhosts outside the provider network—e.g., a network-accessible placementservice may be made available to clients of the VCS, and virtualmachines may be placed at hosts located in the client's own network orat third-party networks using the placement service. In one embodiment,a downloadable or installable software component for generating andenforcing placement policies may be generated, and the softwarecomponent may be used at various computing facilities including privatenetworks located at client premises, networks shared by multipleentities including provider networks, and so on. In addition to beingused for determining where new virtual machines should be launched, inat least some embodiments placement policies may also be used to migrateexisting virtual machines from one host to another.

In various embodiments, a given placement policy may comprise a numberof components or elements, including for example a set of applicabilitycriteria, a set of rules for identifying a pool of candidatevirtualization hosts, and/or a set of rules for ranking the candidatevirtualization hosts to help identify the particular virtualization hostto be used for a particular virtual machine. Several different placementpolicies may apply to a given request for instantiating a virtualmachine in some embodiments, and a combination of the rules indicated inthe different applicable policies may be applied systematically (e.g.,in order of specified precedence priorities) as described below infurther detail. Some policies may be created on behalf of (or by)specific clients or customers, while others may be generated for broaduse on behalf of numerous clients. A number of programmatic interfaces(e.g., web-based consoles, application programming interfaces or APIs,command-line tools, graphical user interfaces or the like) may beimplemented to enable the creation, viewing, comparison or modificationof placement policies in various embodiments.

As mentioned above, in some embodiments placement policies may beenforced by control-plane components of a virtualized computing service(VCS) of a provider network. Generally speaking, networks set up by anentity such as a company or a public sector organization to provide oneor more services (such as various types of multi-tenant and/orsingle-tenant cloud-based computing or storage services) accessible viathe Internet and/or other networks to a distributed set of clients maybe termed provider networks in this document. A provider network mayalso sometimes be referred to as a “public cloud” environment. A givenprovider network may include numerous data centers hosting variousresource pools, such as collections of physical and/or virtualizedcomputer servers, storage devices, networking equipment and the like,needed to implement, configure and distribute the infrastructure andservices offered by the provider. Within large provider networks, somedata centers may be located in different cities, states or countriesthan others, and in some embodiments the resources allocated to a givenapplication may be distributed among several such locations to achievedesired levels of availability, fault-resilience and performance.

The term “control-plane” may be used to refer to administrative ormanagement components of a given provider network service such as theVCS, as opposed to “data-plane” components which may be used primarilyto transmit, store or manipulate client application data. The VCScontrol-plane components responsible for enforcing placement policies,enabling the creation of placement policies and access to placementpolicies, may collectively be referred to herein as the placementmanager of the VCS. A placement manager may comprise numerous softwareand hardware components in some embodiments; that is, the placementmanager may itself be implemented in a distributed manner. In otherembodiments, a placement manager may be implemented in a centralizedmanner, e.g., utilizing a single hardware server. The virtual machineslaunched at VCS virtualization hosts on behalf of clients may bereferred to as “guest virtual machines” or “compute instances”. Inaddition to zero or more guest virtual machines, a given virtualizationhost may also comprise a virtualization management software stack invarious embodiments. The virtualization management software stack mayinclude, for example, a hypervisor and/or an operating system instancerunning on a special privileged-domain virtual machine. Thevirtualization management software stack may act as an intermediarybetween guest virtual machines and hardware devices in variousembodiments.

According to one embodiment, a placement manager of a VCS may receive anindication of a placement policy via a programmatic interface such as aweb-based console, an API (application programming interface), acommand-line tool, or a graphical user interface (GUI). The policy mayinclude one or more applicability criteria, and one or morevirtualization host selection rules. The placement manager may run teststo verify that the placement policy meets acceptance criteria of theVCS—for example, to verify that the submitter of the policy has therequired permissions, to validate the syntax and format of the variouselements of the policy, to ensure that the policy does not violate anypreviously-defined policies of a higher priority, to ensure that thepolicy is implementable given overall constraints on VCS resources, andso on. If the placement policy meets the acceptance criteria, it may beadded to a policy repository of the VCS. If the placement policy failsto meet the acceptance criteria, a rejection notification may beprovided to the submitter of the policy in some embodiments. Therejection notification may indicate the causes of the rejection in someimplementations. Over time, numerous placement policies with respectivesets of applicability criteria and host selection rules may beaccumulated in the policy repository. In various embodiments, at leastsome of the policies may have respective validity periods, e.g., eitherindicated by the policy submitter or determined by the placementmanager. Validity periods may be modified using the programmaticinterfaces of the VCS in some embodiments—e.g., a client may extend orshorten the validity period of a given policy. In such embodiments,expired policies may be pruned from the repository, e.g., by a periodiccleanup thread. In some embodiments, an optimized representation of atleast some subset of the repository's policies may be stored in localpolicy caches at various components of the placement manager, e.g., toavoid having to access the repository when responding to a request tolaunch a guest virtual machine.

In response to a request to launch or instantiate a guest virtualmachine, a VCS control-plane component such as the placement manager mayidentify, based at least in part on one or more attributes or parametersof the launch request, a placement policy or a collection of placementpolicies applicable to the launch request. For example, the launchrequest may explicitly or implicitly indicate the category of guestvirtual machine which is to be launched, the identity of the client onwhose behalf the guest virtual machine is to be launched, some guidanceabout a preferred location for the guest virtual machine, and so on, andthese attributes may be used to identify the group of applicableplacement policies. Using the group of policies, a candidate pool ofvirtualization hosts, typically comprising a subset of the VCS hostfleet, may be identified for the launch request. Some types of hosts maybe excluded from the candidate pool based on specific hardware orsoftware component requirements indicated in the policies, for example.After a candidate pool has been identified, the candidates may be rankedrelative to one another using the policies—e.g., based on factors suchas the network distance (or physical distance) of the candidates fromother guest virtual machines of the same client or from devices of otherprovider network services that the client is expected to use, measuresor estimates of the workload levels of the candidates, and so on. Atarget virtualization host for the requested guest virtual machine maybe identified based on the ranking, and the commands for instantiatingthe guest virtual machine may be executed at the target virtualizationhost. Similarly, when subsequent requests to launch guest virtualmachines are received, the applicable set of policies for each suchrequest may be identified, candidate pools of virtualization hosts maybe populated and ranked, and target virtualization hosts may be selectedfor each request. In some embodiments, respective sets of placementpolicies may be identified and applied at several stages—e.g., aninitial set of placement policies may be identified based on attributesof a launch request, the process of identifying a candidate pool ofvirtualization hosts may be begun using the initial of policies, andadditional placement policies (e.g., data-center-specific policies orother location-specific policies) may be identified and applied asdifferent hosts are considered or ranked as candidates.

In some embodiments, new placement policies may be generatedautomatically (e.g., by various VCS control-plane components) based ontriggering conditions or events. For example, in one embodiment, healthand/or performance metrics may be collected periodically fromvirtualization hosts. If the collected metrics indicate, for example,that resource utilization levels at a particular portion of the VCSnetwork exceed targeted thresholds persistently, a new policy whichreduces the chances of selecting virtualization hosts from that portionof the VCS network may be generated automatically. In anotherembodiment, a database of support requests or client complaints (orother customer feedback) may be maintained for the VCS and/or otherservices of the provider network. If the analysis of the databaseindicates that a particular client has encountered numerous problems ata particular type of virtualization host, a placement policy that avoidsusing that particular type of virtualization host for that client (orfor other clients with similar requirements) may be generatedautomatically.

In at least one embodiment, a client of the VCS may register a set ofexternal virtualization hosts (e.g., hosts located on client-ownedpremises or third-party premises rather than within the providernetwork's data centers) as part of the fleet of hosts for whichplacement decisions are to be made by the VCS placement manager. In suchan embodiment, requests for instantiating virtual machines using theexternal set of virtualization hosts may be transmitted to the VCSplacement manager. The VCS placement manager may identify and enforcethe applicable policies to identify the respective target virtualizationhost from the external set for a given virtual machine request, usinglogic similar to that described above for requests to instantiatevirtual machines using the VCS's own fleet of hosts. In someembodiments, clients may set up their own placement managers, e.g., atclient premises or third party premises, using installable softwarepackages provided by the provider network operator. In one embodiment, agiven placement policy may result in the selection of a provider networkvirtualization host or an external host for a given virtual machine—thatis, policies that cover collections of hosts located in multiplenetworks may be defined and used.

In some embodiments in which multiple placement policies may beapplicable to a particular guest virtual machine request, respectiveprecedence priorities may be associated with the policies. Theprecedence priorities may indicate which of a given pair of policiesoverrides the other. In some embodiments, a set of system-definedpolicies may be designated as having the highest precedence priority (sothat, for example, a client-defined policy may not override asystem-defined policy). Within a single policy, multiple rules may beindicated for identifying candidate hosts or ranking candidate hosts insome embodiments. Respective rule-level precedence priorities may be setfor individual rules in such embodiments. In some embodiments, rules ofa given policy may be defined using Boolean operators to combineindividual rule clauses—e.g., AND, OR, or NOT operators may be used tocombine rules. Precedence priorities at the policy level and/or the rulelevel may be indicated or modified using programmatic interfaces by thepolicy submitters or other authorized entities in various embodiments,and may have to be approved by the placement manager. In at least oneembodiment, an aggregation function indicated in a policy may be used tocombine several different virtualization host ranking metrics—e.g., thearithmetic mean or geometric mean of several different metrics may beused to determine a composite ranking metric.

Numerous categories of guest virtual machines may be supported at avirtualized computing service in different embodiments, with thecategories differing from one another in their hardware and/or softwarecomponents, performance capacity, the kinds of applications for whichthe guest virtual machines are best suited, the pricing modelsassociated with the guest virtual machines, and/or other properties. Invarious embodiments, at least some placement policies may be applicablefor only a subset of guest virtual machine categories. In someembodiments, the applicability criteria for a given policy may indicategroups of guest virtual machines (in addition to or instead ofVCS-defined guest virtual machine categories) to which the policyapplies. For example, some policies may apply to all the guest virtualmachines associated with a given client account, or to an applicationgroup or placement group of a given of the client. A set of guestvirtual machines which are collectively being used for a particularapplication or a set of related applications on behalf of a client maybe defined as an application group by the client. Some clientapplications may run better on operating systems which only run on aparticular subset of virtualization hosts, for example, and placementpolicies that tend to favor the selection of that subset of hosts may becreated and enforced in such scenarios. A placement group may comprise aclient-defined group of guest virtual machines, such that the physicalor network locations of the hosts at which the members of the placementgroup are instantiated meet a condition specified by the client, andsome placement policies may apply only to particular placement groups.In one embodiments, clients may be able to associate tags (e.g., stringschosen by the clients, such as “marketing-department” or “sales”) withone or more guest virtual machines, application groups or placementgroups, and the applicability criteria for a given placement policy mayindicate guest virtual machines to which the placement policy is to beapplied using such tags.

A number of different kinds of location-based applicability criteria forplacement policies may be defined in some embodiments. In oneembodiment, a provider network at which a VCS is implemented may beorganized into a plurality of geographical regions, and each region mayinclude one or more availability containers, which may also be termed“availability zones” herein. An availability container in turn maycomprise portions or all of one or more distinct locations or datacenters, engineered in such a way (e.g., with independent infrastructurecomponents such as power-related equipment, cooling equipment, orphysical security components) that the resources in a given availabilitycontainer are insulated from failures in other availability containers.A failure in one availability container may not be expected to result ina failure in any other availability container; thus, the availabilityprofile of a given resource is intended to be independent of theavailability profile of resources in a different availability container.Various types of services and/or applications may therefore be protectedfrom failures at a single location by launching multiple applicationinstances in respective availability containers. Some location-dependentplacement policies may apply to specified regions, availabilitycontainers, data centers, rooms within data centers, or even toparticular server racks (or types of racks) in various embodiments. Inone embodiment, when a decision is made to shut down hosts located aparticular physical location at some future time (e.g., for extendedmaintenance, repair, or if the location is to be taken offlinepermanently), policies that tend to avoid the selection of hosts at thatphysical location for guest virtual machines that are expected to remainin existence for long periods may be created and enforced.

In some embodiments, the rules used for selecting or excludingvirtualization hosts from a candidate pool may take any of severalfactors into account. For example, some categories of guest virtualmachines may require specific hardware components or software stacks(including, for example, virtualization management software componentssuch as hypervisors, operating systems and the like) which are onlyavailable at a subset of the virtualization hosts of the VCS, so hoststhat do not have the required hardware or software may be eliminatedfrom consideration as candidates. Some virtualization hosts may beexcluded from a candidate pool based on their network locations and/orphysical locations, such as the particular data centers at which thehosts are located. A host which is currently running a particular typeof guest virtual machine may be eliminated from consideration as acandidate for a new guest virtual machine in some embodiments—e.g.,based on single-tenancy requirements associated with the new guestvirtual machine.

Several kinds of ranking rules for candidate virtualization hosts may bespecified in placement policies in various embodiments. For example,some ranking rules may be created to increase the physical spread of agiven client's set of guest virtual machines—e.g., if choosing candidatehost H1 results in a wider physical distribution of the client's fleetof guest virtual machines than choosing candidate H2, H1 would be rankedhigher than H2. A particular client may be very concerned about possibledata loss if any two of their guest virtual machines are launched on thesame host, for example, so a placement policy that strongly ranks hostsbased on physical separation or spread of the client's guest virtualmachines may be used for that client. Various formulas for computingmetrics representing the spread or physical distribution of a givengroup of compute instances may be employed in differentembodiments—e.g., if a pair of a given client's guest virtual machinesare instantiated in respective hosts in separate data centers, a spreadmetric of S1 may be generated, while if the hosts are in separate roomswithin the same data center, a smaller spread metric S2 may begenerated.

Candidate virtualization hosts may also be ranked based on their total(or currently unused) performance capacities, proximity with respect toresources of other provide network services (e.g., a storage servicewhich supports block-device level interfaces, which may be used from theguest virtual machine to be launched for the client), or some measure ofinfrastructure-related costs (e.g., because costs associated withcooling or heating respective groups of virtualization hosts may differ,depending on where the hosts are located). In one embodiment licensingefficiency may be taken into account when ranking candidatevirtualization hosts. For example, some software vendors may licensetheir software at a host level for a minimum time period (e.g., 30days), so if a particular virtualization host H1 has an unexpiredlicense for a software package Pkg1, that host may be ranked higher forcertain types of guest virtual machine requests than a different host H2which does not have such a license. In some embodiments in whichmultiple ranking rules are used, a respective numeric ranking metric maybe generated using each rule. Each of the ranking rules may be assigneda respective weight or strength in such embodiments, and a composite oraggregate ranking metric may be generated (e.g., using a geometric meanor arithmetic mean of weighted individual ranking metrics correspondingto the different ranking criteria). The aggregate ranking metric maythen be used to select a particular virtualization host from among thecandidates.

In various embodiments easy-to-use programmatic interfaces may beimplemented by the placement manager or other components of a VCS toenable clients, VCS administrators and/or other authorized users tocreate, search, view, modify, delete, compare or perform otheroperations on placement policies. Such interfaces may for example enableauthorized users to specify policy applicability criteria, candidatepool selection rules, ranking rules and the like. Logical operationssuch as Boolean ANDS, ORs, NOTs and the like may be used to combinevarious criteria or rule predicates in some embodiments using theprogrammatic interface. The interfaces may be used to assign weights(e.g., to ranking criteria) and/or to specify the specific aggregationfunctions (e.g., arithmetic, geometric or harmonic means) to be used tocombine multiple ranking metrics into an aggregate ranking metric.Precedence criteria for different placement policies may be specified byclients programmatically in some embodiments. The set of supportedinterfaces may include web-based consoles, APIs, command-line tools,graphical user interfaces, and the like in various embodiments.

In response to receiving an indication of a new placement policy or amodified placement policy via such an interface, in various embodimentsthe placement manager may perform one or more validation operations toensure that the new or modified policy meets VCS acceptance criteria(e.g., criteria for syntax correctness, or criteria for compatibilitywith VCS-defined policies which have higher precedence) before savingthe policy in a repository. In at least one embodiment, a client maysubmit a newly-created placement policy with a guest virtual machinelaunch request, or refer to an existing policy in one of the parametersof a launch request. At least some of the placement policies may haveassociated validity periods—e.g., a given placement policy may expireafter some number of days have elapsed since it was activated, unlessits validity period is extended using one of the programmaticpolicy-modification interfaces of the VCS. In addition to being used forlaunching new guest virtual machines, in one embodiment placementpolicies may also be used when migrating existing guest virtual machinesfrom one host to another (e.g., during live migrations in which themigrating guest virtual machine is not rebooted during the migrationprocedure, or during reboot migrations in which the migrating guestvirtual machine is taken offline briefly during the migrationprocedure).

In at least one embodiment, customizable placement policies similar tothose discussed above may be used for instantiating or migrating othertypes of application execution environments in addition to, or insteadof, for instantiating or migrating guest virtual machines. As usedherein, the term “application execution environment” refers to asoftware platform, typically but not necessarily configured on a singlehost, which can be assigned or allocated to a client or an administratorof a network-accessible computing service to enable a desired set ofapplications to be run. Examples of such application executionenvironments may include un-virtualized instances of operating systems,guest virtual machines, administrative-domain virtual machines, and thelike in various embodiments. In one embodiment, some of the clients of acomputing service of a provider network may request allocation ofun-virtualized operating system instances on so-called “bare-metal”hosts (e.g., hosts which do not necessarily have full virtualizationmanagement software stacks installed), and the service may selectappropriate hosts for such operating system instances using placementpolicies of the type discussed above. In some embodiments, much or allof logic that is often implemented at a hypervisor or other componentsof typical virtualization management software stacks may be implementedin hardware devices (such as peripheral devices attached to an I/O bus)of the host instead, and the placement of application executionenvironments on such enhanced hosts may also be managed using placementpolicies. Placement policies may also be used for selecting hosts forapplication execution environments at which administrative components ofvarious services at a provider network are to be run in someembodiments. For example, hosts may be selected using the policies forcustomized or streamlined operating systems at which provider networkrouting applications, load balancing applications, database managementapplications etc. are to be executed.

Example System Environment

FIG. 1 illustrates an example system environment in which customizedplacement policies for guest virtual machines may be used at a virtualcomputing service, according to at least some embodiments. As shown,system 100 includes a provider network 102 at which a number ofnetwork-accessible services including a virtualized computing service(VCS) 105, a storage service 167 and a database service 168 areimplemented. Virtualization host (VH) fleet 172 of VCS 105 may includenumerous virtualization hosts 120 (such as VHs 120A-120G) which maydiffer from one another in various characteristics such as theirhardware architecture, performance capabilities, virtualizationmanagement stack components, supported operating systems, and the like.A given virtualization host may be used to execute one or more guestvirtual machines (GVMs) 130 instantiated in response to launch requests(such as launch request 146) received from clients of the VCS 105. Forexample, GVMS 130A and 130B run at VH 120A, GVM 130C runs at VH 120B,GVM 130D runs at VH 120D, GVM 130F runs at VH 120G, and so on.

The particular virtualization host at which a requested GVM is to belaunched may be selected in the depicted embodiment by a placementmanager 180 using one or more customizable placement policies 165 (e.g.,policies 165A-165C saved in a VCS policy store or repository 190).Placement manager 180 and policy repository 190 may form a subset of thecontrol-plane or administrative resources of the VCS 105 in variousembodiments (as opposed to the data-plane resources of the VCS, whichmay include the GVMs 130 and may be used primarily to process, transmit,or store client application data).

The placement manager 180 may implement one or more sets of programmaticinterfaces 133 in the depicted embodiment for interactions with GVMrequesting clients 145 and authorized placement policy creators 144. Theprogrammatic interfaces may include, for example, one or more web-basedconsoles, APIs, command-line tools, graphical user interfaces and thelike. In some implementations, separate sets of programmatic interfacesmay be provided for policy creators 144 and GVM requesters 145.

An authorized policy creator 144 (e.g., an administrator of the VCS orone of the other services of the provider network 102, or a privilegedclient of the VCS) may submit a new placement policy using theprogrammatic interfaces, or edit an existing policy 165. Policy editingand validation modules 184 of the placement manager may execute one ormore tests to ensure that the proposed new or modified policy meetsacceptance criteria of the VCS in the depicted embodiment. If the policyis deemed acceptable, it may be added to the policy store 190;otherwise, the policy creator may be informed that the policy isunacceptable. To determine whether a given policy is acceptable, in someembodiments the editing/validation modules 184 may first check that thepolicy is formatted according to a supported syntax. For example, in oneembodiment the VCS may enable potential policy creators to obtain apolicy schema specification document (e.g., via the VCS programmaticinterfaces) which defines the particular languages or formats (e.g., aversion of the Extensible Markup Language or XML, or a version of theJavascript Structured Object Notation or JSON format) in which placementpolicies can be expressed, required and optional subcomponents ofpolicies, the defaults corresponding to the optional subcomponents ofpolicies, VCS rules or meta-policies which cannot be violated, and soon. If the policy passes the syntax check, the compatibility of thepolicy with respect to previously-defined policies (which may includeone or more inviolable policies established by the VCS) may be checkedin some embodiments, and the policy may be rejected if it contradicts orviolates a higher-level or higher-priority policy. The policy may alsobe checked for implementation feasibility—e.g., if the policy requiresthat each of a given client's GVMs be launched at a separate datacenter, the VCS may not be able to enforce the policy for more than afew GVMs and so may reject the policy.

A given policy 165 may include, among other elements such as thosediscussed below in the context of FIG. 2, applicability criteriaindicating the kinds of GVMs or VHs to which it applies, a VH candidatepool selection rule set, and a VH candidate ranking rule set. Uponreceiving a particular launch request 146 (e.g., for a GVM 130E) withrequest attributes 147 from a client 145, policy enforcement modules 186of the placement manager may identify a set of one or more policies 165which are applicable to the launch request 146 in the depictedembodiment. The attributes 147 of the request may, for example,implicitly or explicitly indicate the identity of the client on whosebehalf the GVM 130 is to be instantiated, the category of the requestedGVM (e.g., selected by the requester from a catalog of GVM categoriessupported at the VCS 105), the availability container or data center atwhich the GVM is to be launched, and so on. The policy enforcementmodules 186 may identify the relevant placement policies 165 based atleast in part on one or more request attributes 147 and theapplicability criteria defined for the policies in the depictedembodiment. A number of different kinds of applicability criteria may beused in various embodiments, such as applicability based on clientidentity, VH location, GVM category, application group membership orplacement group membership, etc., as described below in further detail.In some embodiments as also discussed below, a cached version of a setof placement policies 165 may be maintained at the placement manager, sothat placement decisions may be made more efficiently than if querieshad to be directed to policy store 190 for each launch request. It isnoted that the placement manager 180 may itself be implemented in adistributed manner in at least some embodiment, with respective policyenforcement modules executing at different hardware servers.

A candidate pool 152 of virtualization hosts may be identified for therequest 146 in the depicted embodiment, based at least in part on thecandidate pool selection rule sets of the applicable policies. In someembodiments, in scenarios in which multiple placement policies apply toa given request 146, the rules in the policies may be enforced inaccordance with respective precedence priorities associated with thepolicies. For request 146 pertaining to GVM 130E, candidate VH pool 152comprises VHs 120E, 120F and 120G. It is noted that although a smallcandidate pool has been shown in FIG. 1 due to space limitations, inpractice a candidate pool may comprise hundreds or thousands ofvirtualization hosts in at least some embodiments. After the members ofcandidate pool 152 have been identified, they may be ranked relative toone another using the ranking rule sets of the applicable placementpolicies in the depicted embodiment. Ranking rules may be designed basedon various factors as discussed in detail below, such as the spread orphysical distribution of the requesting client's set of GVMs, proximityof the requested GVM to resources of a different service (which islikely to be used from the requested GVM) such as storage service 166 ordatabase service 167, licensing efficiency, and so on. In oneimplementation respective numeric ranking metrics may be calculated foreach applicable ranking rule of a given policy, and the individualmetrics may be combined into an aggregate ranking metric using acombination technique indicated in the policy. If multiple policiesapply, the ranking rules of the policies may be applied in order ofprecedence priorities in some embodiments. In one embodiment, respectiveweights may be assigned to the different ranking rules of a given policy(or to ranking rules of different applicable policies) and the weightsmay be used to arrive at an aggregate ranking metric for a givenvirtualization host. The results of the ranking may be used to identifythe target virtualization host for the requested GVM. In the depictedexample, VH 120F is selected as the target for GVM 130E based on itsranking among the members of pool 152. The GVM 130E may be launched atthe target virtualization host, and the client 145 may be notified viathe programmatic interfaces 133 that the GVM is available for use.

The VCS control-plane may also include a fleet manager 173 and a GVMmigration manager 182 in the depicted embodiment. The fleet manager 173may be responsible for implementing high-level VCS strategies thattypically affect large numbers of the VHs (or the entire VH fleet 172)over relatively long time periods—e.g., decisions to gradually deprecateone type of hardware host in favor of another may be made by the fleetmanager 173, or decisions to set the overall goal for host utilizationlevels may be made by the fleet manager 173. In at least someembodiments, at least a subset of the placement policies 165 may begenerated to implement decisions made at the fleet manager level. TheGVM migration manager 182 may be responsible for deciding, based onvarious factors, whether to move existing GVMs from their currentvirtualization hosts to different virtualization hosts. The GVMmigration manager 182 may, for example, determine that a set of GVMs isto be transferred or migrated (e.g., because their current hosts are toooverloaded, being deprecated, or need to be taken offline formaintenance reasons), and use one or more of the placement policies 165to select the target virtualization hosts for the migrations. A numberof different types of placement policy-based migrations may be supportedin various embodiments, including for example live migrations (in whichthe migrating GVM does not need to be rebooted during the migrationprocedure) or reboot migrations (in which the migrating GVM is stoppedand restarted during the migration procedure).

In at least some embodiments, one or more of the placement policies 165may be generated automatically by VCS control-plane components based onchanging conditions in the VCS fleet. For example, respective sets ofhealth metrics associated with the various virtualization hosts of thevirtualized computing service may be collected by a health monitoringservice 158 and/or by the fleet manager 173 of the VCS control-plane.Based at least in part on the collected health metrics for some subsetof the host fleet, and on various service quality objectives of the VCS,new placement policies 165 may be created or removed over time, e.g., bythe health monitoring service, the placement manager or the fleetmanager. In one scenario, if the health metrics indicate that theutilization levels of some collection of VHs consistently exceed athreshold, a policy that tends to avoid those hosts may be created andapplied, e.g., at least for some duration until the utilization levelsfall below the thresholds. In another embodiment, a database of userexperience records 162 (which may be managed by a customer supportorganization of the VCS) may be used to generate new placement policiesautomatically. For example, if a given client C1 repeatedly encountersproblems with their applications when the applications are run on aparticular stack of VH hardware or software, policies to avoid thatstack of VH hardware or software for C1's future launch requests may becreated automatically by the placement manager or some othercontrol-plane component. In at least some embodiment, some of theplacement policies 165 may be generated, applied and/or deleted withoutnotifying the clients whose GVMs are affected by the policies. Someclients may not necessarily be aware of the use of placement policies invarious embodiments.

In one embodiment, a client may submit a launch request 146 whichincludes or indicates a new placement policy which has not previouslybeen transmitted to or accepted by the placement manager. In such ascenario, the new placement policy may be analyzed by the placementmanager for acceptability (e.g., by checking the syntax of the policyand/or the compatibility of the policy with previously-definedhigher-priority policies as discussed above). If the new policy isacceptable, it may be used (in combination with any other placementpolicies which may be applicable) to select the target virtualizationhost for the launch request. Depending on the preferences of the launchrequest submitter, the policy may or may not be saved in the policyrepository maintained by the placement manager. Thus, in some cases, agiven placement policy may not necessarily be used more than once.

In some embodiments, placement decisions for application executionenvironments other than guest virtual machines may also or instead bemade using placement policies 165. For example, customizable placementpolicies may be created and enforced for placing operating systeminstances (to be used for client applications and/or administrativeapplications of various provider network services) on hosts which do notnecessarily have virtualization management software components such ashypervisors installed. In one embodiment, some un-virtualizedapplication execution environments may be instantiated on (or migratedto) hosts outside the provider network based on the applicable placementpolicies.

Placement Policy Components

FIG. 2 illustrates example elements of a placement policy for guestvirtual machines, according to at least some embodiments. Placementpolicy 200 may include some combination of a policy schema version 201,policy name 203, an identifier 205 of the policy creator or owner, avalidity period 207, applicability criteria 209, a virtualization hostcandidate pool selection rule set 211, a candidate ranking rule set 213,a precedence priority 215, access permissions 217, and parent/childpolicy pointers 219 in the depicted embodiment. As mentioned earlier, insome embodiments a VCS may publish a schema specification indicating theacceptable languages or formats (e.g., versions of XML, JSON, etc.) inwhich policies are to be expressed and/or stored. The schemaspecification may change over time, e.g., as features such as new GVMcategories are added to the VCS, and the particular version of theschema with which a given policy complies may be indicated via schemaversion element 201 in the depicted embodiment. The policy name 203 maycomprise a string selected by the creator of the policy, such as“Web-app-W1 policy” or the like. The entity responsible for creating thepolicy (e.g., either by modifying a pre-existing policy, or byindicating various elements of the policy without using a pre-existingpolicy as a template) may be indicated in the creator/owner identifierelement 205. For example, for client-created policies, the VCS clientaccount identifier or a user identifier associated with a particular VCSclient may be used for element 205, while for policies generated by VCSadministrators, a generic identifier such as “system” may be used insome implementations.

The validity period 207 may indicate, for example, a start date ortimestamp at which the policy is to be made effective, and an end dateor timestamp after which the policy is no longer to be enforced.Validity periods may be set, extended or shortened by authorizedentities using the VCS programmatic interfaces in variousembodiments—e.g., an authorized client may decide to extend a validityperiod of a policy which appears to be working well. Applicabilitycriteria 209 may indicate the kinds of GVM requests to which the policyis to be applied; examples of different applicability criteria arediscussed below in the context of FIG. 3, FIG. 4 and FIG. 5. Rule sets211 and 213 may indicate factors which are to be considered whengenerating a candidate pool of virtualization hosts, and when rankingthe candidates after the pool has been formed, respectively; examples ofsuch factors are discussed below in the context of FIG. 6.

In some embodiments, several different policies may apply to a givenlaunch request or to a given virtualization host, and respectiveprecedence priorities 215 to be used to resolve conflicts (if any) amongthe policies may be stored together with the other elements of thepolicies. In one embodiment, for example, a group of system-definedpolicies may be designated as high-priority policies which cannot beviolated by policies created by individual clients. Such system-definedpolicies may be assigned a high precedence priority value of 1 (assuminglower integers correspond to higher priorities) in one implementation,for example, while clients may request precedence priorities with valuesin the range 2-10000. A given client may create multiple placementpolicies including P1 and P2, with P1 being considered more importantfrom the client's perspective than P2. In such a scenario, the clientmay assign a precedence priority 10 to P1 and a priority 20 to P2. Inone implementation, precedence priorities may be used to indicatewhether a given policy is a requirement (from the policy creator'sperspective) or a “nice-to-have” feature with respect to a given set ofGVMs or hosts.

Access permissions 217 may be used to indicate the set of entities orusers who are allowed to view and/or modify the policy. In someembodiments, the permissions may be defined and assigned using anidentity management system implemented at the provider network. In oneembodiment, at least some placement policies may be related to eachother hierarchically—e.g., one policy P-root may be used as the templatefrom which two other more specific policies P-child1 and P-child2 aregenerated, where the children policies differ from the parent in asubset of elements. In such scenarios, the relationships between thedifferent policies of the hierarchy may be indicated using child orparent pointers 219. For example, instead of duplicating all the commonelements of parent policy P-root in P-child1 and P-child-2, a pointer toP-root may be stored in the children policies, together with anyelements that differ from the parent policy. It is noted thatsystem-defined defaults may be used for one or more placement policyelements which are not explicitly specified by the policy creator invarious embodiments—for example, a default validity period of 30 daysmay be set for a policy for which no explicit validity period isspecified. In various embodiments, placement policies with one or moreelements not shown in FIG. 2 may be implemented, or one or more of theillustrated elements may not be included in at least some placementpolicies.

Applicability Criteria Examples

FIG. 3 illustrates examples of location-based applicability criteria forplacement policies, according to at least some embodiments. In thedepicted embodiment, the resources of VCS 305 are distributed among aplurality of availability containers (ACs) such as AC 320A and 320B. Asmentioned earlier, the resources in a given availability container 320may be engineered such that they are insulated from failures in otheravailability containers. Because a failure in one availability containeris not expected to result in a correlated failure in any otheravailability container, fault-tolerant applications may sometimes beimplemented suing redundant sets of components located in differentavailability containers. A given availability container may comprise atleast a respective portion of one or more data centers. In the depictedexample, AC 320A includes data centers 330A and 330B (which maythemselves be geographically distributed to improveintra-availability-container fault-tolerance), while AC 320B includesdata center 330C. Each data center 330 may in turn comprise one or morerooms, such as rooms 340A and 340B within data center 330A, and rooms340K and 340L within data center 330C. Each room may contain numerousserver racks, such as racks 350A and 350B in room 340A, and racks 350Land 350M in room 340K. Each rack may be used for a set of virtualizationhosts, such as VH set 360A at rack 350A, VH set 360B at rack 350B, VHset 360V at rack 350L, and VH set 360W at rack 350M. In the depictedembodiment, various components or instances of the placement manager(such as the policy enforcement modules 186 shown in FIG. 1) may bedistributed among the availability containers and the data centers—e.g.,placement manager components 374 may be configured at the availabilitycontainer 320A, and placement manager components 375 may be set up atdata center 330C.

As indicated in table 360, location-based applicability criteria 390 maybe defined for placement policies at various granularities in thedepicted embodiment. For example, placement policy P1 applies to GVMs atall virtualization hosts of availability container 320A, regardless ofthe specific rack, room, or data center at which the hosts are located.In contrast, placement policy P2 applies to hosts located in data center330B of availability container 320A, and may not be applicable to hostslocated in other data centers such as 330A of the same availabilitycontainer. Such location-specific policies may be particularly usefulwhen decisions to decommission or shut down specific data centers, roomsor racks, either temporarily or permanently, are made at the VCS. It isnoted that some location-based policies may be identified for a givenlaunch request (or a given migration operation) only after someindication of the location of the candidate hosts is obtained by theplacement manager. For example, in some embodiments each GVM launchrequest may be associated with a corresponding availability container,in which case the set of placement policies associated with thatavailability container may be included in the set relevant policies forany launch request for which that availability container is to be used.

In some embodiments a VCS may allow a client to specify, in a launchrequest, a selected category from among numerous supported categories ofguest virtual machines, and the placement policies identified for thelaunch request may depend on the category specified by the client. FIG.4 illustrates examples of supported categories of guest virtual machinesfor which respective placement policies may be generated, according toat least some embodiments. In the depicted embodiment, GVM categorycatalog 404 of a VCS includes a number of different families of guestvirtual machines. Each family consists of a plurality of categories ofguest virtual machines; the members of a given family may in some casesall be optimized for particular kinds of applications but may differfrom one another in performance capacity, billing rates, and the like. Astandard GVM category family 410 comprises general-purpose GVMcategories “small”, “medium”, and “large” (with the names reflectingrelative performance capability levels), suitable for various commonapplications such as web hosting, software development, officeapplications and the like. GVMs belonging to the standard family 410 maytypically be configured on commodity virtualization hosts which do notrequire any custom hardware in the depicted embodiment.

GVM categories FPO-A and FPO-B of floating-point-optimized family 420may be suited for high-end applications involving intensive scientificcomputations, such as applications associated with astronomysimulations, physics simulations and the like. Some applications mayrequire very high rates of I/O (input-output) operations, and instancesof the I/O-optimized GVM category family 430 (such as category IO-A orIO-B) may be established for VCS clients with such applications.Applications which can benefit from using graphical processing units(GPUs) may be deployed at guest virtual machines belonging to GPU-basedGVM category family 440, such as GPU-A or GPU-B categories. Additionalcategory families beyond those shown in FIG. 2 may be supported at someVCSs in various embodiments, and some of the category families indicatedin FIG. 4 may not be available in some embodiments.

As indicated in Table 460, some placement policies may be applicable toselected GVM categories 490. For example, policy P15 applies to all thecategories of the standard GVM category family 410, while policy P22applies to GVMs belonging to the GPU-A category of family 440. In atleast one embodiment, default placement policies may be defined at theVCS for each supported GVM category, with clients allowed to override atleast some of the default policies (e.g., by assigning higher precedencepriorities to the non-default policies).

FIG. 5 illustrates examples of placement policies applicable toclient-specific collections of guest virtual machines, according to atleast some embodiments. The guest virtual machines of VCS 502 may belogically subdivided into a number of groups or collections, with eachcollection being associated with at least one VCS client in the depictedembodiment. The GVMs allocated to three different clients C1, C2 and C3are shown. Within a given client's GVM collection, a number ofsub-groups may be defined by the client. An application group may bedefined by a client in some embodiments, e.g., as a label which can beindicated as an attribute of a launch request, to refer to all the GVMsbeing used at various tiers of a particular application. For example,client C2 has organized GVM set 510B into application groups 522A and522B. Application group 522A includes GVMs 530K, 530L and 530M, whileapplication group 522B includes GVMs 530D, 530E and 530F.

In one embodiment, clients may be able to specify, for a set of GVMsdesignated as members of a placement group, a desired property of aphysical location of one GVM of the placement group relative to thephysical location of another GVM of the placement group. For example,for some clustered applications in which the latency or bandwidth ofmessages transmitted from one cluster node to another has to meet aparticular criteria, a placement group which requires different clusternodes to be located no more than X meters from one another may bedefined. When requesting a launch of a new GVM, a placement group forthe new GVM may be specified as a request attribute in some embodiments.In the depicted example, client C3's GVMs 530P, 530Q, 530R and 530S allbelong to a bandwidth-based placement group 524. The placement of theGVMs of placement group 524 may be further constrained in the depictedembodiment, in that the virtualization hosts selected for the placementgroup members may have to be linked to one another using specialhigh-bandwidth links 555 which may only be available at some datacenters of the provider network.

In the depicted embodiment, clients may also assign arbitrary tags (suchas tag T1) to one or more GVMs (such as GVM 530U or 530V), and the setof GVMs to which a particular tag is assigned may form anotherdistinguishable GVM collection for which a particular placement policymay be defined. Some clients, such as C1 in the depicted example, maynot create sub-groups such as application groups or placement groups oftheir GVMs, and may not utilize tags; in such cases, client-specificplacement policies may be applicable to all the GVMs established onbehalf of the client.

In the depicted embodiment, four examples of placement policies whoseapplicability is limited to client-specific GVM collections 590 areshown in table 560. Policy P51 is applicable to GVM set 510A, comprisingall the GVMs of client C1. Policy P52 applies to application group 522Aof client C2. Policy P53 applies to GVMs with which tag T1 is associated(e.g., GVMs 530U and 530V). Placement policy P54 is applicable toplacement group 524 defined by client C3.

The types of applicability criteria shown in FIG. 3, FIG. 4 and FIG. 5may be combined in some embodiments. For example, a given placementpolicy may be applied to all of client C1's GVMs which belong tostandard GVM category “small” and are instantiated within a particularavailability container 320A in one embodiment, combining aspects ofclient-specific applicability, location-based applicability, and GVMcategory-based applicability. In at least some embodiments, additionaldimensions of applicability may be defined for some placement policies.For example, in embodiments in which temporal applicability issupported, a particular placement policy P-k may be applied to those ofa client's launch requests which are received during one specified timeperiod on any given day (e.g., between 8 AM and 5 PM), and a differentplacement policy P-1 may be applied to requests received outside thattime period (e.g., between 5 PM and 8 AM). In some embodiments,different placement policies may be defined based on the total number ofGVMs allocated currently to a client—e.g., policy P-s may apply to aclient on whose behalf less than one thousand GVMs have beenestablished, and policy P-t may apply to clients on whose behalf morethan one thousand GVMs have been set up.

Candidate Host Selection and Ranking

FIG. 6 illustrates examples of factors that may be taken into account toselect members of candidate pools of virtualization hosts for placing aguest virtual machine, and to rank members of such candidate pools,according to at least some embodiments. In some embodiments, therationale for selecting a particular host as a candidate for a GVM withspecified characteristics or requirements may be based on whether it iseven possible for a GVM with those characteristics to be instantiatedsuccessfully at that host, so the process of identifying candidate hostsmay comprise eliminating or excluding hosts that cannot support therequested GVM regardless of the possible advantages or disadvantages ofdoing so. Some inclusion or exclusion rules 602 for virtualization hostcandidate pools may be based on host hardware components attributes 604in some embodiments. Thus, some categories of guest virtual machines mayonly be compatible with certain hardware architectures—e.g., a minimumof N1 processor cores with clock speeds of G1 GHz, and supporting a64-bit architecture may be a prerequisite for launching a GVM ofcategory Cat1, while N2 cores running at clock speeds of G2 GHz andsupporting a 32-bit architecture may be required for category Cat2 GVMs.Attributes of the virtualization management software stack 606 installedat a given virtualization host may also determine whether than host canbe considered a candidate. For example, some GVMs may require aparticular version of a hypervisor or a privileged domain operatingsystem to be running at the virtualization host, and an attempt tolaunch such a GVM at a host with the wrong version of the hypervisor orthe wrong operating system version may simply fail. Some GVMs may haveto be launched using a particular virtual machine image, which may notbe readable or executable at a subset of the virtualization host fleet,thereby eliminating that subset of hosts from the candidate pool forsuch GVMs.

In some cases, some types of GVMs may not be co-located at a host atwhich a GVM of another type (or the same type) is currently running—forexample a GVM of category Cat1 may be incompatible with a GVM ofcategory Cat2 because both categories may attempt to hold on to a sharedresource such as a GPU for long time slots. Thus, attributes 610 ofother GVMs running at a given host may disqualify that host fromconsideration as a candidate. In one embodiment, the VCS may allowclients to request single tenancy for a GVM—that is, a requested GVM mayonly be launched at a host at which no other GVM is running.Single-tenancy requirements may eliminate at least some hosts fromconsideration as candidates. In at least one embodiment, it may not befeasible to launch a given GVM at a particular data center—for example,GVMs of that category may be intended to remain online for months oryears, but the data center may have been selected for decommissioning inthe next few weeks. In another example of data center attributes 608that may be taken into account when selecting candidate virtualizationhosts, a given data center may not have sufficiently-fast network linksto be able to sustain the traffic levels associated with a requestedGVM. In one embodiment, a limit may be placed on the resources that areto be consumed in selecting candidate VHs for a GVM launch request.Candidate search resource consumption limits 612 may, for example, limitthe maximum number of virtualization hosts that are to be considered aspossible candidates for a given GVM launch request to say, 10000 or100000 in some embodiments, and may therefore also influence theexclusion or inclusion of hosts in the candidate pool.

Factors that may be evaluated to rank different candidate hosts for aGVM according to candidate ranking rules 652 may include, for example,the “spread” or physical distribution 654 of the requesting client'sGVMs. For availability and failure tolerance reasons, some clients maywish to maximize the physical separation of their GVMs to the extentpossible, based on the assumption that the probability of correlatedfailures at two hosts may vary inversely with the distance between thetwo hosts. The relative importance of the different ranking factors tobe used to select a target host for a GVM may be indicatedprogrammatically by the policy creator in some embodiments—for example,for client C1, increasing or maximizing spread may be a much higherpriority than it is for client C2, so respective policies with rankingrules that assign different weights to maximizing spread may be used forC1 and C2. Licensing efficiency 656 may be taken into account whenranking candidate hosts for certain types of GVMs in the depictedembodiment. With respect to some types of software licenses (e.g.,licenses from third-party vendors of database software or otherapplication software), in some cases each license may have a minimumterm (e.g., a year) and may be designated for a specific host. Whendeciding where to place a given GVM which is expected to use a licensedsoftware product LSP-1, those hosts for which a currently unused orvalid license for LSP-1 exists may be ranked higher than hosts which donot LSP-1 licenses. A host may have an unused license because, forexample, a different GVM that required such a license may have beenrunning earlier on that host, and that different GVM may have beenterminated before the license term expired. In various embodiments, agroup of candidate virtualization hosts may be ranked so as to maximizethe use of existing licenses which have been obtained for some or all ofthe candidates.

Proximity 658 with respect to resources of other services of theprovider network may also be used to rank virtualization hosts in someembodiments. For example, in one embodiment, a given GVM may be expectedto use storage objects (e.g., a volume exposing a block deviceinterface) from a particular storage service of the provider network,and those virtualization hosts which happen to be located in closeproximity with a volume-providing storage device of that service may bepreferred for storage performance reasons in the ranking procedure.Proximity to components of other services of a provider network, such asa database service or a load balancing service, may also be taken intoaccount to rank the hosts in various embodiments.

In some embodiments, when ranking candidate hosts, recent measurementsof trends in network workload levels (e.g., based on trafficmeasurements or network link utilization measurements) in the networkvicinity of the hosts may be used to rank hosts as more suitable or lesssuitable for a given GVM. Based on such host neighborhood networkworkload trends 666, a host in a quieter or less-trafficked portion ofthe VCS network may be ranked higher than hosts in busier portions ofthe network. Recent host-level resource utilization and/or performancemetrics 660 may be used for ranking rules 652 in some embodiments. Theelapsed time 662 since a host's most recent maintenance window (e.g.,since the host was most recently rebooted for maintenance) may be takeninto account as a ranking factor in some embodiments, e.g., under theassumption that hosts which have been rebooted more recently are lesslikely to be rebooted again in the near future and are thereforepreferable. In one embodiment, metrics 664 of infrastructure costs(e.g., costs associated with heating, cooling, physical security and thelike) may be kept for different sets of virtualization hosts, and thehosts which are likely to incur lower infrastructure costs may be rankedhigher. In at least some embodiments, respective numerical rankingmetrics may be computed for each ranking factor for a given host, andcombined to obtain an aggregate ranking metric. The aggregate metrics ofdifferent candidates may be compared to identify the best-suited targethost for a given GVM.

Example Programmatic Interface

As mentioned earlier, the placement manager may implement a number ofprogrammatic interfaces in some embodiments for interactions withplacement policy creators. FIG. 7 illustrates an example web-basedprogrammatic interface which may be used to generate or edit placementpolicies for guest virtual machines, according to at least someembodiments. As shown, the interface comprises a web page 700 includingan introductory message area 705 and a graphical editing tool 707. Themessage area provides an overview of the purpose of the web page 700 anda link to a tutorial video about the creation and management ofplacement policies.

Within editing tool 707, web controls 709 may be used to generate a newpolicy or to edit an existing policy to which the user has been grantedthe appropriate access permissions. Element 711 identifies the policywhich is currently being edited. One or more applicability criteria forthe policy may be specified in region 713. For each criterion, one ormore clauses or predicates may be specified, and multiple clauses may becombined using Boolean operators such as “AND” or “OR” in the depictedembodiment. A given clause may contain one element indicating arespective attribute or property (e.g., “GVM category” or “Availabilitycontainer”), a relational operation (e.g., “==”, signifying equality, or“!=” signifying inequality), and a value or value range (e.g., a valueof “Small” for the “GVM category” property, and “AC-US-East-0045” forthe “Availability container” property) may be indicated using theediting tool 707.

In region 715 of web page 700, a client may specify virtualization hostcandidate selection criteria or rules. In the depicted example, criteriafor including a virtualization host in the candidate pool for requestsassociated with the “Small” category of GVMs in the availabilitycontainers which meet the inequality criterion of region 713 may includethat the virtualization host have the hypervisor version HV1 or HV2installed. As in the case of the applicability criteria, multiplecriteria for inclusion in the candidate pool may be combinable usingBoolean operators in the depicted embodiment.

With respect to ranking candidate pool members, region 717 of web page700 may be utilized to provide a list of ranking metrics, and control719 may be used to indicate how the numerical values corresponding tothe individual ranking metrics are to be combined into a singleaggregated ranking metric. In the depicted example, a client hasselected ranking metrics for GVM spread, licensing efficiency andproximity to a storage service SS1, and indicated that the geometricmean of the individual metrics is to be used as the aggregate metric.Web control 721 may be used to save the policy being edited or created.In various embodiments, drop-down menus or other controls may beprovided to allow clients to select entries to be included in theapplicability criteria, the candidate selection criteria or the rankingcriteria. Additional controls may be provided to indicate validityperiods, precedence priorities, access permissions and/or other elementsof placement policies in some embodiments. Instead of or in addition toa web-based interface similar to that shown in FIG. 7, APIs,command-line tools, or graphical user interfaces that do not compriseweb pages may be provided in some embodiments for placement policycreation, viewing and editing. Programmatic tools to compare existingpolicies (e.g., the equivalent of a “diff” command) may also besupported in some embodiments at the VCS.

Policy Caching

When a launch request for a GVM is received at the VCS control-plane, acorresponding placement decision may have to be made fairly quickly invarious embodiments. Caching of policies within main memory may be usedto speed up the real-time responses of policy enforcement modules of theplacement manager in some embodiments. In various embodiments, theresponsibilities of placement policy enforcement may be distributedamong a plurality of nodes of the placement manager: for example,respective placement manager nodes may be established in everyavailability container or in every data center. FIG. 8 illustrates theuse of policy caches at nodes of a distributed placement manager,according to at least some embodiments.

Approved placement policies 865, such as 865A, 865B or 865C mayinitially be placed in a policy repository 890 in the depictedembodiment. Periodically or on demand, an optimized policy setrepresentation (e.g., respective tree-structured representations 851Aand 851B) may be generated from at least a subset of the stored policies865 and transmitted to in-memory caches located at respectivedistributed placement manager components. In the depicted embodiment,respective policy caches 844A and 844B may be maintained in main memoryat the servers at which data-center level placement manager components880A and 880B are executed. The set of policies cached at differentcomponents 880 may not be identical—e.g., a set of policies relevant tothe corresponding data center may be sent to each data-center levelplacement manager component. In the depicted example, policies P1-P5 arestored in local cache 844A, while policies P1, P7, P9, P8 and P5 arestored in local cache 844B. The tree structured representations maycorrespond to decision trees that have to be traversed to make placementdecisions—e.g., P1 may be evaluated and applied first at component 880A,then P2 may be applied if it is relevant to the request beingconsidered, and so on. In some implementations, respective policyindexes such as IX1 or IX2 may be generated and provided to thedistributed placement manager components. Such indexes may speed up theprocess of searching for policies based on client ID, GVM category, andso on. In some embodiments an executable program corresponding to one ormore placement policies may be pre-compiled at the policy repository orat some other component of the VCS, and transmitted to the distributedplacement policy components 880. For example, a byte-code version of theset of placement policies may be created in one implementation. At agiven distributed placement manager component 880, such an executableversion may be run with a given launch request's attributes provided asinput parameters. The executable version of the policies may provide aranked candidate virtualization host list as its output, and/or identifya particular virtualization host to be used for the request.

Placement Decisions for Hosts Outside the Provider Network

In some embodiments, techniques for enforcing policy-drive placementpolicies of the kind described above may be utilized for avirtualization host fleet which comprises at least some hosts locatedoutside the provider network. FIG. 9 illustrates an examplepolicy-driven placement manager which, while implemented using resourcesof a provider network, may be used for placement decisions at a clientnetwork, according to at least some embodiments. As shown, the fleet ofavailable virtualization hosts from which VCS-based placement manager980 can select includes a VCS fleet partition 972 and a client-ownedfleet partition 973. VCS fleet partition 972 comprises VHs 920A and920B, which may be located in one or more data centers of providernetwork 902, while client-owned fleet partition 973 comprisesclient-owned virtualization hosts 921A and 921B located within clientnetwork 914. In the depicted embodiment, a client may provide varioustypes of information about the set of client-owned virtualization hostsfor which placement decisions are to be managed by the VCS placementmanager, including for example network addresses, administrativecredentials, and the like. In one embodiment the VCS-based placementmanager 980 may run a suite of validation tests to verify that it canperform the operations required to launch virtual machines at theclient-owned fleet partition 973. The results of two example placementdecisions (which may have been made on behalf of the same VCS client, oron behalf of different VCS clients) made by policy enforcement modules986 are illustrated in FIG. 9. In placement decision 955A,virtualization host 920A of the VCS is selected for GVM 930A based onapplicable policies, while in placement decision 955B, virtualizationhost 921A at client network 914 is selected for GVM 930B based onapplicable policies.

In some embodiments in which external virtualization hosts are managedwith the help of a placement manager 980 implemented as part of anetwork-accessible service of the provider network, one of theattributes of a launch request for a GVM may indicate whether the GVM isto be instantiated at a VCS VH 920 or a client-owned VH 921. In otherembodiments, the decision as to whether a VCS VH 920 is to be used orwhether a client-owned VH is to be used may be made by the placementmanager 980 based on policies 965. In some embodiments in which multipleGVMs can be requested in single launch request, one or more applicablepolicies may result in the selection of some hosts at the VCS and somehosts at the client network for the same launch request. The policyediting and validation modules 984 and the policy repository 990 mayboth be implemented using VCS resources in the depicted embodiment.Different clients may specify respective fleet partitions for which GVMplacement decisions are to be made using the VCS placement manager 980in some embodiments. In one embodiment, virtualization hosts located inthird-party premises (e.g., data centers that are owned or managed byentities other than the provider network operator and other than VCSclients) may be included in the fleet from which target virtualizationhosts can be identified by the placement manager. In some embodiments,the placement manager may expose programmatic interfaces enablingauthorized clients and/or third parties to extend or modify the policycreation and implementation algorithms, at least with respect to asubset of the virtualization host fleet partitions. For example, in oneembodiment in which such an extensible framework for placement policiesis supported, a particular client may customize the core placementmanager logic, and use the customized placement manager for the client'sown virtualization hosts or for a fleet that includes some VCS hosts andsome non-VCS hosts. In at least one embodiment, a client may utilize aVCS-based placement manager 980 for client-premise virtualization hostsalone (i.e., for a fleet which does not include virtualization hostswithin provider network 902).

In contrast to the scenario illustrated in FIG. 9, in some embodimentsall the components of a placement manager may be implemented or executedoutside the provider network. FIG. 10 illustrates an examplepolicy-driven placement manager which may be executed using resources ofa client network, according to at least some embodiments. The placementmanager logic may be packaged in the form of an executable program whichcan be installed on client-owned computing devices, for example. Thepolicy editing and validation modules 1084 and the enforcement modules1086 may be executed at client-owned computer servers, and the contentsof policy repository 1090 may be stored at storage devices of clientnetwork 1014 in the depicted embodiment. In the depicted embodiment, apolicy-based decision 1055 made by the enforcement modules 1086 hasresults in the launch of GVM 1030 on client-owned VH 1021A of VH fleet1073.

In the placement manager implementation mode illustrated in FIG. 10, theplacement policies 1065 (e.g., 1065A or 1065B) generated and used fordifferent clients may be extensively customized based on the clients'specific placement-related goals and requirements. For example, thekinds of request attributes which are used to select applicable policiesat one client network may not overlap at all with the kinds of requestattributes used at another client's network, and completely differentfactors may be used by different clients for virtualization hostcandidate selection and ranking.

Methods for Supporting Customized Placement Policies

FIG. 11 illustrates aspects of operations that may be performed tosupport customized policies for selecting virtualization hosts forvirtual machines, according to at least some embodiments. As shown inelement 1101, an indication of a placement policy Ppol1 to be used toselect virtualization hosts for guest virtual machines may be receivedvia a programmatic interface, e.g., at a placement manager of a virtualcomputing service of a provider network. PPol1 may include a number ofelements, including for example applicability criteria, a set of rulesfor determining members of a pool of candidate virtualization hosts,and/or a set of rules or criteria for ranking the candidates relative toeach other. In some embodiments, one or more of the elements of PPol1may be specified using a web-based programmatic interface, a set ofAPIs, a command-line tool or a GUI.

A number of validation operations may be performed to ensure that Ppol1meets one or more acceptance criteria (e.g., the syntax in which thepolicy elements are specified may be checked with respect to a supportedpolicy schema, the compatibility of PPol1 with respect to system-definedpolicies which cannot be overridden may be verified, and so on) (element1104). PPol1 may be stored in a repository of accepted policies if itmeets the acceptance criteria. Optimized representations of at least asubset of the repository's policies may be distributed among variousplacement management components responsible for responding to GVM launchrequests in some embodiments. For example, some subset or all of thepolicies may be cached in the local memories accessible from variouspolicy enforcement modules at which run-time placement decisions aremade in some embodiments. In one embodiment, an executable version(e.g., a byte-code version, or a compiled version) of relevant placementpolicies may be cached at various computing devices used for policyenforcement.

When the next GVM launch request is received at a particular policyenforcement component (element 1107), a set of placement policiesincluding Ppol1 that are applicable to the request may be identified(element 1110), e.g., based at least in part on the policies'applicability criteria and/or on request attributes such as therequesting client's identity, the type of GVM requested, and so on.Using the applicable policies, a pool of candidate virtualization hostsfor the request may be identified (element 1113). From the candidatepool, a particular virtualization host may be selected as the target forthe requested GVM based on the ranking rules indicated in the applicablepolicies (element 1116). The requested GVM may be launched at the targethost (element 1119). Operations corresponding to elements 1107 onwardsmay be repeated for various GVM launch requests. In some embodiments, inaddition to being used for selecting hosts for new GVMs, the placementpolicies may also be used to identify destinations to which existingGVMs should be migrated, e.g., using either reboot migration or livemigration.

It is noted that in various embodiments, some of the operations shown inFIG. 11 may be implemented in a different order than that shown in thefigure, or may be performed in parallel rather than sequentially.Additionally, some of the operations shown in FIG. 11 may not berequired in one or more implementations.

Use Cases

The techniques described above, of enabling the creation and enforcementof customizable placement policies for guest virtual machines may beuseful in a variety of environments. Large virtualized computingservices may serve hundreds of thousands of clients, for each of whomnumerous guest virtual machines may have to be launched using resourcesdistributed among dozens of data centers spread around the world. Theplacement needs and preferences of some clients may differ substantiallyfrom those of others, and the equipment and operating conditions ofdifferent portions of the service network may change over time. Forexample, various sets of virtualization hosts may need to be takenoffline eventually for maintenance or other reasons without disruptingexisting long-running client applications; placement policies that tendto avoid the selection of such deprecated hosts for new virtual machinesmay help to gradually move client workloads away from the deprecatedhosts. Separating the specification of placement rules from theenforcement logic for those rules may help to simplify theimplementation of guest virtual machine launch operations considerably.Placement policies that can be set and modified (e.g., by editing XMLdocuments or JSON objects) without requiring code changes to thecontrol-plane of the virtual computing service may provide substantialflexibility with respect to meeting capacity management and resourceutilization goals.

Illustrative Computer System

In at least some embodiments, a server that implements one or more ofthe techniques described above for supporting customizable placementpolicies (including for example the operations performed by placementmanagers, virtualization hosts, and other control-plane or data-planecomponents of a virtualized computing service) may include ageneral-purpose computer system that includes or is configured to accessone or more computer-accessible media. FIG. 12 illustrates such ageneral-purpose computing device 9000. In the illustrated embodiment,computing device 9000 includes one or more processors 9010 coupled to asystem memory 9020 (which may comprise both non-volatile and volatilememory modules) via an input/output (I/O) interface 9030. Computingdevice 9000 further includes a network interface 9040 coupled to I/Ointerface 9030.

In various embodiments, computing device 9000 may be a uniprocessorsystem including one processor 9010, or a multiprocessor systemincluding several processors 9010 (e.g., two, four, eight, or anothersuitable number). Processors 9010 may be any suitable processors capableof executing instructions. For example, in various embodiments,processors 9010 may be general-purpose or embedded processorsimplementing any of a variety of instruction set architectures (ISAs),such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitableISA. In multiprocessor systems, each of processors 9010 may commonly,but not necessarily, implement the same ISA. In some implementations,graphics processing units (GPUs) may be used instead of, or in additionto, conventional processors.

System memory 9020 may be configured to store instructions and dataaccessible by processor(s) 9010. In at least some embodiments, thesystem memory 9020 may comprise both volatile and non-volatile portions;in other embodiments, only volatile memory may be used. In variousembodiments, the volatile portion of system memory 9020 may beimplemented using any suitable memory technology, such as static randomaccess memory (SRAM), synchronous dynamic RAM or any other type ofmemory. For the non-volatile portion of system memory (which maycomprise one or more NVDIMMs, for example), in some embodimentsflash-based memory devices, including NAND-flash devices, may be used.In at least some embodiments, the non-volatile portion of the systemmemory may include a power source, such as a supercapacitor or otherpower storage device (e.g., a battery). In various embodiments,memristor based resistive random access memory (ReRAM),three-dimensional NAND technologies, Ferroelectric RAM, magnetoresistiveRAM (MRAM), or any of various types of phase change memory (PCM) may beused at least for the non-volatile portion of system memory. In theillustrated embodiment, program instructions and data implementing oneor more desired functions, such as those methods, techniques, and datadescribed above, are shown stored within system memory 9020 as code 9025and data 9026.

In one embodiment, I/O interface 9030 may be configured to coordinateI/O traffic between processor 9010, system memory 9020, networkinterface 9040 or other peripheral interfaces such as various types ofpersistent and/or volatile storage devices. In some embodiments, I/Ointerface 9030 may perform any necessary protocol, timing or other datatransformations to convert data signals from one component (e.g., systemmemory 9020) into a format suitable for use by another component (e.g.,processor 9010). In some embodiments, I/O interface 9030 may includesupport for devices attached through various types of peripheral buses,such as a Low Pin Count (LPC) bus, a variant of the Peripheral ComponentInterconnect (PCI) bus standard or the Universal Serial Bus (USB)standard, for example. In some embodiments, the function of I/Ointerface 9030 may be split into two or more separate components, suchas a north bridge and a south bridge, for example. Also, in someembodiments some or all of the functionality of I/O interface 9030, suchas an interface to system memory 9020, may be incorporated directly intoprocessor 9010.

Network interface 9040 may be configured to allow data to be exchangedbetween computing device 9000 and other devices 9060 attached to anetwork or networks 9050, such as other computer systems or devices asillustrated in FIG. 1 through FIG. 11, for example. In variousembodiments, network interface 9040 may support communication via anysuitable wired or wireless general data networks, such as types ofEthernet network, for example. Additionally, network interface 9040 maysupport communication via telecommunications/telephony networks such asanalog voice networks or digital fiber communications networks, viastorage area networks such as Fibre Channel SANs, or via any othersuitable type of network and/or protocol.

In some embodiments, system memory 9020 may be one embodiment of acomputer-accessible medium configured to store program instructions anddata as described above for FIG. 1 through FIG. 11 for implementingembodiments of the corresponding methods and apparatus. However, inother embodiments, program instructions and/or data may be received,sent or stored upon different types of computer-accessible media.Generally speaking, a computer-accessible medium may includenon-transitory storage media or memory media such as magnetic or opticalmedia, e.g., disk or DVD/CD coupled to computing device 9000 via I/Ointerface 9030. A non-transitory computer-accessible storage medium mayalso include any volatile or non-volatile media such as RAM (e.g. SDRAM,DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc., that may be included in someembodiments of computing device 9000 as system memory 9020 or anothertype of memory. Further, a computer-accessible medium may includetransmission media or signals such as electrical, electromagnetic, ordigital signals, conveyed via a communication medium such as a networkand/or a wireless link, such as may be implemented via network interface9040. Portions or all of multiple computing devices such as thatillustrated in FIG. 12 may be used to implement the describedfunctionality in various embodiments; for example, software componentsrunning on a variety of different devices and servers may collaborate toprovide the functionality. In some embodiments, portions of thedescribed functionality may be implemented using storage devices,network devices, or special-purpose computer systems, in addition to orinstead of being implemented using general-purpose computer systems. Theterm “computing device”, as used herein, refers to at least all thesetypes of devices, and is not limited to these types of devices.

CONCLUSION

Various embodiments may further include receiving, sending or storinginstructions and/or data implemented in accordance with the foregoingdescription upon a computer-accessible medium. Generally speaking, acomputer-accessible medium may include storage media or memory mediasuch as magnetic or optical media, e.g., disk or DVD/CD-ROM, volatile ornon-volatile media such as RAM (e.g. SDRAM, DDR, RDRAM, SRAM, etc.),ROM, etc., as well as transmission media or signals such as electrical,electromagnetic, or digital signals, conveyed via a communication mediumsuch as network and/or a wireless link.

The various methods as illustrated in the Figures and described hereinrepresent exemplary embodiments of methods. The methods may beimplemented in software, hardware, or a combination thereof. The orderof method may be changed, and various elements may be added, reordered,combined, omitted, modified, etc.

Various modifications and changes may be made as would be obvious to aperson skilled in the art having the benefit of this disclosure. It isintended to embrace all such modifications and changes and, accordingly,the above description to be regarded in an illustrative rather than arestrictive sense.

What is claimed is:
 1. A system, comprising: one or more control-planecomponents of a virtualized computing service, wherein the one or morecontrol-plane components are executed at one or more computing devicesof a provider network; wherein the one or more control-plane componentsare configured to: receive, via a programmatic interface, a plurality ofindications of a plurality of placement policies for guest virtualmachines, including a first placement policy, wherein the firstplacement policy includes (a) one or more policy applicability criteriaand (b) one or more virtualization host selection rules; in response toverifying that the first placement policy meets an acceptance criterionof the virtualized computing service, store a representation of thefirst placement policy in a repository, wherein the repository comprisesthe plurality of placement policies; and in response to receiving alaunch request for a particular guest virtual machine, select, based atleast in part on one or more attributes of the launch request, a groupof one or more placement policies, from the plurality of placementpolicies in the repository, applicable to the launch request, whereinthe group of one or more placement policies includes the first placementpolicy, and wherein indications of the placement policies of the groupof one or more placement policies were received prior to and separatelyfrom the launch request; determine, using the group of one or moreplacement policies, a candidate pool comprising one or morevirtualization hosts of the virtualized computing service for theparticular guest virtual machine; determine a ranking, using the groupof one or more placement policies, of the one or more virtualizationhosts of the candidate pool; and instantiate the particular guestvirtual machine at a target virtualization host selected from thecandidate pool based at least in part on the ranking of the one or morevirtualization hosts of the candidate pool.
 2. The system as recited inclaim 1, wherein the one or more control-plane components are configuredto: obtain respective sets of health metrics associated with the one ormore virtualization hosts of the virtualized computing service; generatea second placement policy based at least in part on (a) the respectivesets of health metrics and (b) one or more service quality objectives ofthe virtualized computing service.
 3. The system as recited in claim 1,wherein the one or more control-plane components are configured to:obtain one or more user experience records associated with respectiveclients of the virtualized computing service; generate a secondplacement policy based at least in part on an analysis of the one ormore user experience records.
 4. The system as recited in claim 1,wherein the one or more control-plane components are configured to:receive a second launch request for a second guest virtual machine,wherein the second launch request includes an indication of a secondplacement policy which has not been transmitted earlier to thevirtualized computing service; and select a virtualization host for thesecond guest virtual machine based at least in part on the secondplacement policy.
 5. The system as recited in claim 1, wherein the firstplacement policy has a first precedence priority, wherein the group oneor more placement policies includes a second placement policy with asecond precedence priority, wherein the second placement policy is asystem-generated policy, wherein the first placement policy is generatedin response to a policy creation request from a client of the virtualcomputing service, wherein the second precedence priority is higher thanthe first precedence priority, and wherein the one or more control-planecomponents are configured to: utilize the respective precedencepriorities of the first and second placement policies to (a) determinethe candidate pool and (b) rank the virtualization hosts of thecandidate pool.
 6. A method, comprising: performing, by one or morecontrol-plane components of a virtualized computing service: receiving,via a programmatic interface, a plurality of indications of a pluralityof placement policies for guest virtual machines, including a firstplacement policy, to be instantiated at the virtualized computingservice, wherein the first placement policy includes one or morevirtualization host selection rules; selecting, based at least in parton one or more attributes of a launch request for a particular guestvirtual machine, a group of one or more placement policies, from theplurality of placement policies in a repository of placement policies,that are applicable to the launch request, wherein the group of one ormore placement policies includes the first placement policy, and whereinindications of the placement policies of the group of one or moreplacement policies were received prior to and separately from the launchrequest; determining, using the group of one or more placement policies,a candidate pool of one or more virtualization hosts of the virtualizedcomputing service for the particular guest virtual machine; determininga ranking, using the group of one or more placement policies, of the oneor more virtualization hosts of the candidate pool; and instantiatingthe particular guest virtual machine at a target virtualization hostselected from the candidate pool based at least in part on the rankingof the one or more virtualization hosts of the candidate pool.
 7. Themethod as recited in claim 6, further comprising performing, by the oneor more control-plane components: storing a representation of the firstplacement policy in a persistent repository of the virtualized computingservice; loading, into a cache from the repository prior to receiving anindication of the launch request, a representation of the one or moreplacement policies of the group applicable to the launch request;wherein said identifying the one or more placement policies comprisesexamining the cache.
 8. The method as recited in claim 6, wherein thegroup of one or more applicable policies includes a second placementpolicy, further comprising performing, by the one or more control-planecomponents: storing an indication of a precedence priority of the firstplacement policy relative to the second placement policy, wherein theprecedence priority is determined based at least in part on a source ofthe first placement policy, and wherein the precedence priority isutilized for one or more of: (a) determining the candidate pool or (b)ranking virtualization hosts of the candidate pool.
 9. The method asrecited in claim 6, wherein the source of the first placement policy isa client of the virtualized computing service, and wherein the source ofthe second placement policy is an entity other than a client, andwherein the precedence priority of the second placement policy is higherthan the precedence priority of the first placement policy.
 10. Themethod as recited in claim 6, wherein the virtualized computing servicesupports instantiation of guest virtual machines of a plurality ofcategories including a first category and a second category, wherein aperformance capacity of a guest virtual machine of the first categorydiffers from a performance capacity of the a guest virtual machine ofthe second category, wherein the indication of the first placementpolicy includes an applicability criterion of the first placementpolicy, wherein the applicability criterion indicates that the firstplacement policy is applicable to one or more of: (a) a particularcategory of the plurality of categories of guest virtual machines, (b) aparticular collection of virtualization hosts whose resource utilizationstatus meets a specified criterion, (c) a particular availabilitycontainer of a provider network, (d) a particular data center, (e) acollection of guest virtual machines allocated to a particular client,wherein members of the collection meet a specified criterion.
 11. Themethod as recited in claim 10, wherein the specified criterion met bymembers of the collection of guest virtual machines allocated to theparticular client comprises one or more of: (a) membership in aplacement group created for the particular client, wherein a property ofa physical location of one guest virtual machine of the placement grouprelative to another guest virtual machine of the placement group isspecified by the client, (b) membership in an application group createdfor the particular client, wherein individual ones of the guest virtualmachines of the application group are used for respective components ofa particular application, or (c) an association of a client-generatedtag with individual ones of the collection of guest virtual machines.12. The method as recited in claim 6, wherein the one or morevirtualization host selection rules include a particular rule to rankmembers of the candidate pool of virtualization hosts based on one ormore of: (a) a metric indicative of a physical distribution of a set ofguest virtual machines instantiated on behalf of a particular client,(b) a performance metric, (c) a licensing efficiency metric, (d) ametric indicative of proximity with respect to one or more resources ofa network-accessible service other than the virtual computing service,or (e) an infrastructure cost metric.
 13. The method as recited in claim12, wherein the service other than the virtual computing servicecomprises one or more of: (a) a storage service, (b) a database service,or (c) a load balancing service.
 14. The method as recited in claim 6,wherein the one or more virtualization host selection rules include aparticular rule to exclude, from the candidate pool of virtualizationhosts, one or more of (a) a virtualization host whose virtualizationmanagement software stack meets a specified criterion, (b) avirtualization host which includes a hardware component meeting aspecified criterion, (c) a virtualization host installed at a specifieddata center, or (d) a virtualization host at which a guest virtualmachine meeting a specified criterion is instantiated.
 15. The method asrecited in claim 6, further comprising performing, by the one or morecontrol-plane components: receiving, via the one or more programmaticinterfaces, a request to modify a precedence priority of the firstplacement policy relative to a different placement policy; and storingan indication of a modified precedence policy of the first placementpolicy.
 16. A non-transitory computer-accessible storage medium storingprogram instructions that when executed on one or more processors:receive a plurality of indications of a plurality of placement policies,including a first placement policy, for application executionenvironments to be instantiated using a fleet of hosts, wherein thefirst placement policy includes one or more host selection rules; inresponse to receiving a request for a particular application executionenvironment, identify a group of one or more placement policies from theplurality of placement policies applicable to the particular applicationexecution environment, wherein the group includes the first placementpolicy, and wherein indications of the placement policies of the groupof one or more placement policies were received prior to and separatelyfrom the request; determine, using the group of one or more placementpolicies, a candidate pool of one or more hosts of the fleet for theparticular application execution environment; determine a ranking, usingthe group of one or more placement policies, of the one or more hosts ofthe candidate pool; and instantiate the particular application executionenvironment at a target host selected from the candidate pool based atleast in part on the ranking of the one or more hosts of the candidatepool.
 17. The non-transitory computer-accessible storage medium asrecited in claim 16, wherein the instructions when executed at the oneor more processors: receive, via the programmatic interface, a requestto edit the first placement policy; generate an edited version of thefirst placement policy in accordance with the request to edit; andperform one or more validation operations on the edited version, andstore the edited version of the first placement policy in a persistentrepository after the one or more validation operations have succeeded.18. The non-transitory computer-accessible storage medium as recited inclaim 17, wherein the request to edit the first placement policycomprises an indication of a validity period of the first placementpolicy.
 19. The non-transitory computer-accessible storage medium asrecited in claim 16, wherein the instructions when executed at the oneor more processors: determine that a second application executionenvironment, instantiated on a first host of the fleet, meets amigration criterion; and select, based at least in part on the firstplacement policy, a second host to which the second applicationexecution platform environment is to be migrated.
 20. The non-transitorycomputer-accessible storage medium as recited in claim 16, wherein thefleet of hosts comprises at least one host located in a data center of aprovider network, and at least one host located at premises owned by aclient of the provider network.